In order to protect a copyright of a content, more specifically to prevent a malicious use of a content, such as unauthorized playing back or copying of the content, a cryptography technology is commonly used and an encrypted content is distributed.
Only an authorized device can hold or obtain a decrypting key to decrypt and play back such an encrypted content.
A cryptography function is often hardware-implemented in a device in view of the processing speed and tamper-resistance. Meanwhile, in order to secure the safety, there is a demand for changing a cryptosystem implemented in the device to another when the cryptosystem is broken.
The patent document 1 discloses a technique that enables a cryptosystem to be updated with use of a reconfigurable hardware device such as a FPGA (Field Programmable Gate Array) and a PLA (programmable logic Array). Also, the patent document 2 discloses a technique in which predetermined encrypting algorithm files are stored in a database, and one of the algorithm files are selected from the database and updated according to an external instruction.    Patent document 1: Japanese Laid-Open Patent Application Publication No. H10-320191    Patent document 2: Japanese Laid-Open Patent Application Publication No. H10-55135